1. Field of the Invention
This invention relates to a machining system wherein a variety of workpieces are machined by plural machining units arranged along a transfer path on which the workpieces are transferred.
2. Prior Art of the Invention
Conventionally, a transfer machine has been used for efficiently machining a large number of workpieces. As shown in FIG. 1, such transfer machine is provided with plural single purpose machining units M1-M4 which are installed at plural machining stations C1-C4 located along a transfer path on which plural pallets holding workpieces are successively transferred by a transfer apparatus 3 for machining the workpieces. Further, when some of workpieces require special machinings which cannot be carried out by the single purpose machining units M1-M4, additional special machining units M10-M12 are installed at additional machining stations S1-S3 for carrying out the special machinings. The entrance end and exit end of the transfer apparatus 3 are connected through a pallet return conveyers 2a, 2b and 2c. A workpiece exchange station E is arranged in the midway of the pallet return conveyer 2b.
In such transfer machine, its machining cycle time is determined based upon the production speed of the workpieces, and the machining at each of machining stations C1-C4 and S1-S3 must be completed within the predetermined machining cycle time. Therefore, in cases where part of workpieces require a special machining which takes very long time to complete, the special machining must be divided into plural machinings so as to make the machining time of each divided machining operation shorter than the machining cycle time. In such case, several single purpose machining units M10-M12 are installed for the special machinings, whereby the length of the transfer machine becomes long, and the installation space and installation cost of the transfer machine also increase. Especially, in cases where only small number of workpieces require such special machinings, the installation of a large number of special machining stations is not economical.
Recently, a flexible transfer machine has been developed for efficiently machining plural kinds of workpieces. In such flexible transfer machine, each single purpose machining unit is composed of a numerically controlled machining unit which is provided with a single kind of tool but memorizes plural kind of machining programs corresponding to the kinds of workpieces to be machined. The kind of a workpiece is detected at each machining station to select a machining program corresponding to the kind of the workpiece, whereby the selected machining program is executed to machine the workpiece.
Such flexible transfer machine also has problems similar to those of the above-mentioned conventional transfer machine. Even though each single purpose machining unit can store plural kinds of machining programs, the kinds of machinings are limited by the kind of tool provided in the single purpose machining unit, because each single purpose machining unit of such flexible transfer machine is provided with a single kind of tool. Therefore, when some of workpieces require special machinings which cannot be carried out by the single purpose machining units, additional single purpose machining units must be installed to carry out the special machinings. When the special machinings take long time to be carried out, the special machinings are divided into plural machinings so that each of single purpose machining units can carry out each of divided machining operations within a predetermined machining cycle time. This results in an increase of the number of additional single purpose machining units for special machinings, thereby causing an increase of the installation space and cost of the transfer machine.
Further, when a malfunction, for example, the breakage of a tool occurs in one of single purpose machining units, the production of workpieces must be stopped. This hinders the production schedule of the workpieces.